The effectiveness of beach mega-nourishment, assessed over three management epochs

Resilient coastal protection requires adaptive management strategies that build with nature to maintain long-term sustainability. With increasing pressures on shorelines from urbanisation, industrial growth, sea-level rise and changing storm climates soft approaches to coastal management are implemented to support natural habitats and maintain healthy coastal ecosystems. The impact of a beach mega-nourishment along a frontage of interactive natural and engineered systems that incorporate soft and hard defences is explored. A coastal evolution model is applied to simulate the impact of different hypothetical mega-nourishment interventions to assess their impacts’ over 3 shoreline management planning epochs: present-day (0–20 years), medium-term (20–50 years) and long-term (50–100 years). The impacts of the smaller interventions when appropriately positioned are found to be as effective as larger schemes, thus making them more cost-effective for present-day management. Over time the benefit from larger interventions becomes more noticeable, with multi-location schemes requiring a smaller initial nourishment to achieve at least the same benefit as that of a single-location scheme. While the longer-term impact of larger schemes reduces erosion across a frontage the short-term impact down drift of the scheme can lead to an increase in erosion as the natural sediment drift becomes interrupted. This research presents a transferable modelling tool to assess the impact of nourishment schemes for a variety of sedimentary shorelines and highlights both the positive and negative impact of beach mega-nourishment

Modelling Large-Scale CO2 Leakages in the North Sea

A three dimensional hydrodynamic model with a coupled carbonate speciation sub-model is used to simulate large additions of CO2 into the North Sea, representing leakages at potential carbon sequestration sites. A range of leakage scenarios are conducted at two distinct release sites, allowing an analysis of the seasonal, inter-annual and spatial variability of impacts to the marine ecosystem. Seasonally stratified regions are shown to be more vulnerable to CO2 release during the summer as the added CO2 remains trapped beneath the thermocline, preventing outgasing to the atmosphere. On average, CO2 injected into the northern North Sea is shown to reside within the water column twice as long as an equivalent addition in the southern North Sea before reaching the atmosphere. Short-term leakages of 5000 tonnes CO2 over a single day result in substantial acidification at the release sites (up to -1.92 pH units), with significant perturbations (greater than 0.1 pH units) generally confined to a 10 km radius. Long-term CO2 leakages sustained for a year may result in extensive plumes of acidified seawater, carried by major advective pathways. Whilst such scenarios could be harmful to marine biota over confined spatial scales, continued unmitigated CO2 emissions from fossil fuels are predicted to result in greater and more long-lived perturbations to the carbonate system over the next few decades

Hydrodynamic timescales in a hyper-tidal region of freshwater influence

This study uses a three dimensional hydrodynamic model to investigate transport timescales in Liverpool Bay, a shallow hyper-tidal region of freshwater influence (ROFI) with a density-driven baroclinic residual circulation. Flushing time, residence time and age are evaluated, providing rigorously defined parameters to describe the rate of offshore freshwater transport and basin replenishment. Additional challenges encountered when assessing these timescales in a tidally mixed regime are highlighted by idealised models. Climatological river gauge data reveals that the numerous local rivers contribute an average of 203 m3 s−1 of freshwater to Liverpool Bay. Based upon the mean salinity distribution, this would suggest a flushing time of approximately 136 days. The mean residence time of the region is approximately 103 days although small concentrations of water are retained over several years due to vigorous tidal mixing. Age in the region is highly variable with regular oscillations caused by tidal advection, whilst long term fluctuations are governed by river flow rates. The mean age gradient is directed offshore, approximately parallel to both the salinity gradient and the major axis of the tidal ellipse, with basin wide average magnitude of 6 days km−1. It is shown that salinity may be used to estimate the age of freshwater, which is not directly observable in practice

Behaviour influences larval dispersal in shelf sea gyres: Nephrops norvegicus in the Irish Sea

The western Irish Sea seasonal gyre is widely believed to play an important role in the local retention of resident larvae. This mechanism could be particularly crucial for the larvae of the heavily fished crustacean Nephrops norvegicus (L.), as their sediment requirements highly restrict where they are able to settle. As recent research suggests that the gyre may be becoming less retentive due to changes in atmospheric forcing, it is now crucial to understand how the gyre influences dispersal. This investigation addresses the hypothesis that shelf sea gyres reinforce larval retention using a biophysical model with vertical migration, habitat selection and temperature-dependent pelagic larval duration (PLD) configured to match the behaviour of N. norvegicus larvae. The results of this study suggest that the gyre does increase the likelihood that passive larvae remain within the western Irish Sea, on the condition that the larvae remain fixed at the depth of peak gyral flow. Retention rates are significantly lower when vertical migration is introduced, and there is no evidence that the gyre promotes larval retention amongst either vertically migrating larvae, or larvae that require muddy sediments for successful settlement. By contrast, vertical migration is shown to be favourable for retention in the eastern Irish Sea. PLD varies by a factor of two according to release date and location. The simulations suggest that whilst some highly limited and almost entirely unidirectional larval exchange may occur, the distinct sites largely rely upon local recruitment

Spatio-temporal variability in the tipping points of a coastal defense

To enable effective adaptive management, early warning of when a ‘tipping point’ within a system’s defense may occur is vital. A tipping point is a critical threshold at which the state of a system is altered, perhaps irreversibly. After the extremity of the UK’s 2013/2014 winter, many coastal systems have undergone a change in state. For example, the conversion of a sandy beach into a rocky platform or an increase/decrease in flood hazard due to a defense breach or new intervention. Coastal monitoring networks around the UK have enabled data collection of these extreme events to drive model applications to assess plausible changes in coastal conditions that trigger a sudden change in a system’s state and conditions that enable recovery. Using available UK monitoring networks and a numerical approach, we focus on Dungeness and Rye Bay, a region of high value in terms of habitat and energy, to assess (i) how the natural variability within the profile of the gravel barrier modifies the overwash rates that can occur and (ii) how ambitious human intervention that re-scape the geomorphic character of the shoreline could impact the critical point at which overwash occurs